Inventions disclosed herein pertain to shaping sheet glass and sheet thermoplastic materials through application of heat to soften the sheet and use of a fluid or gas to form it. As used herein, the term thermoplastic refers to any material whose viscosity changes as a function of temperature, including silicate glasses such as borosilicate glass and fused silica, and polymeric materials such as polymethyl methacrylate (PMMA).
Glass sheets are used in a wide range of applications, such as flat panel displays, hard disk drives, car windshields, decorative ornaments, and optics used in x-ray telescopes. Different applications require different tolerances on the final shape of the sheets. The flat panel display and space telescope industries require thin sheets to minimize weight, yet substrates used in such applications require tight tolerances on their thickness uniformity and surface flatness in order to meet the required specifications for each field.
In the case of liquid crystal flat panel displays, two glass sheets are separated by a small gap of less than 10 μm in thickness, whereby this gap is filled by the liquid crystal. In order to prevent display mura such as color mura or contrast mura and to impart good display performances such as uniform display, high contrast ratio, and wide viewing angle to the color liquid crystal display, one has to maintain the gap between the two glass sheets to be constant and uniform. This becomes a challenge when the glass sheets used have wavy surfaces which result in a variation in the gap size.
The shaping of glass and thermoplastic materials by applying heat and allowing the material to sag by gravity into a mold or mandrel is well known. This process is also known as slumping. The temperature of the workpiece is raised close to its softening point, and the heated sheet sags by gravity to conform to the shape of a mold, which can be of any general shape. Some methods use forces other than gravity, such as vacuum or a moveable plunger onto the surface of the thermoplastic sheet, to facilitate and accelerate the shaping process. In this process it is important that the mold be fabricated of a material which is able to withstand a temperature above the softening temperature of the sheet without damage or significant deformation. After the desired shape has been achieved, the sheet and mold are slowly cooled below the softening temperature of the sheet resulting in the solidification of the thermoplastic material into the desired shape.
Slumping onto mandrels of any shape and whose surface has been ground and polished to a desired tolerance can be used as a method to improve the surface flatness of thermoplastic sheets. Glass sheets manufactured using different processes, such as the float process, the fusion process and the slot-draw process, are commonly used as substrates for flat panel displays and space telescopes. The slot-draw and fusion processes have minimal or no contact between the glass sheet and any tools as the glass is formed. This is particularly important for the flat panel display industry because it eliminates the introduction of impurities or damage to the glass sheet, leaving its surface pristine and smooth and of a fire-polished quality.
Slumping such glass or thermoplastic sheets onto a mandrel to change the shape of the sheet or to improve its flatness can compromise the quality of the sheet's surface. Contact with the mandrel at the elevated temperatures required for the process of slumping can mar the surface of the sheet. The presence of dust particles and other particulate impurities or thin contaminant materials sandwiched between the sheet and the mandrel can result in ripples in the final surface of the sheet after slumping. The removal of all dust particles and contaminants from the surface of the mandrel and the thermoplastic sheet can be difficult in a manufacturing environment.
In some cases, the complete removal of dust particles, for example by thorough cleaning and use of a clean room environment, can result in fusion of the mandrel with the workpiece being formed. For example, see Ralf K. Heilmann et al., Novel methods for shaping thin-foil optics for x-ray astronomy, Proc. SPIE, Vol. 4496 (2001). Fusion of the mandrel with the workpiece can also result from a slumping procedure which is excessively long or proceeds at too high of a temperature. This problem is well known to practitioners of the art of glass and thermoplastic molding. For example, glass artisans commonly apply a thin coating to the mold made of a slurry of fine refractory particles to prevent sticking during slumping. For another example, thin coatings of organic release agents are commonly applied to molds during the forming of thermoplastic polymers to prevent sticking. For another example, graphitic release agents are commonly applied to molds used during the forming of glass automobile windshields. In all these cases, however, the coating's roughness and the aforementioned effects of particulates and other surface contaminants can compromise the figure and surface quality of the final product.
Inventions disclosed herein eliminate the deleterious effects of contact by the workpiece with the mandrel by introducing a thin film of moving fluid, such as air, between the sheet and the mandrel. The mandrel thus acts as a fluid bearing with the bearing fluid being a liquid or a gas. It is important to select a fluid that can withstand the high temperatures needed for the process, which temperatures must exceed the softening temperature of the thermoplastic material. For example, for the case of polymeric materials, the fluid may be water or oil. For the case of a borosilicate glass, the fluid may be a gas such as air or nitrogen.
It is also important that the gap between the mandrel and the sheet be controlled to a dimension that is larger than the typical maximum size of particulate impurities in the manufacturing environment. This condition allows the thin layer of fluid to envelop or absorb the impurity particles, thus preventing such particles or surface contaminants from transmitting forces between the mandrel and workpiece. The fluid layer also provides the required force to shape the surface of the glass or thermoplastic material.
A primary object of inventions disclosed herein is to overcome the aforementioned disadvantages of known processes used for shaping sheet glass or sheet thermoplastic materials.
Another object of inventions disclosed herein is to shape sheet glass or sheet thermoplastic materials on a fluid bearing by using the force from a moving layer of fluid, such as air sandwiched between the sheet and the reference mandrel. The fluid bearing can be made of porous material or of machined or fabricated materials.